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Resistance to water uptake by conifer seedlings Dosskey, Michael Gordon


Water availability for uptake by tree seedlings is determined both by the soil water potential in relation to seedling needle water potential and by the resistance to flow of water through the soil, root and stem, to the needles. This study was designed to focus principally on water uptake resistances. The effects of soil texture and tree species on this water uptake resistance were quantified through the use of an Ohm's Law model suited to water flow through the soil-plant system. The study was conducted on one-year-old potted seedlings in a controlled environment growth chamber. Needle water potential (ΨN) of Douglas-fir is not much affected by soil water potential (Ψs) down to about -2.5 MPa, where the calculated water uptake rate becomes very small. However, soil texture does significantly affect the resistance to flow into the seedling and thus affects the water uptake rate by the seedling. The total resistance to water uptake increases as the soil dries. Coarser textured soils show consistently higher water uptake resistances over the soil water potential range -0.5 to -2.5 MPa. It is inferred that differences in resistance are associated with unsaturated, hydraulic conductivity characteristics of the soil and soil-root contact. Unlike Douglas-fir, both western and mountain hemlock show a large decrease in needle water potential as the soil dries down to a Ψs of about -3.0 MPa. The water potential difference (Ψs - ΨN) for hemlocks is less where Ψs is higher than -1.8 MPa, and greater where Ψs is less than -1.8 MPa, than (Ψs - ΨN ) for Douglas-fir in these s s N experiments. Despite these differences, the resistance to water uptake for both hemlock species is much greater over the soil water potential range -0.5 to -2.5 MPa, and thus the water uptake rates are much less than for Douglas-fir with the same soil, even though root densities and root surface areas are much larger for the hemlocks. This behavior is most pronounced with mountain hemlock. These differences are thought to be related to higher tissue and (perhaps) soil-root contact resistances in the hemlock species. The soil resistance appears to be small, at least down to Ψs of about -2.0 MPa, in these experiments. However, root densities are probably much greater than one might expect in the field.

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